This invention relates in general to techniques for separating a selected gas from a mixture of gases and, more particularly, to an apparatus and method for separating oxygen from other gases by employing electrolytic systems.
Electrolytic systems such as fuel cells and electrolyzers are known. Fuel cells typically include cathode and anode electrodes situated in, and separated by, an electrolyte. The electrolyte may be either solid or liquid. In contrast to batteries which actually store electrical energy in a chemical form, fuel cells are structures in which the reactants and the reactant products are continuously supplied and removed to produce electricity. More specifically, a fuel and an oxidant are continuously supplied to the fuel cell so as to react and thereby directly produce an electrical potential between the cathode and anode electrodes. Thus, a fuel cell is generally regarded as being an energy converter whereas a battery is considered to be an energy source.
Electrolyzers, like fuel cells, also employ cathode and anode electrodes situated in an electrolyte. However, in an electrolyzer a source of DC current is supplied between these electrodes such that the electrolyte is separated or decomposed into its component ions via electrolytic action.
Devices which electrolytically concentrate gases are related in general principle to the fuel cell and the electrolyzer technologies discussed above. Fuel cells or electrolyzers are generally classified according to the type of electrolyte which they use to sustain electrolytic conduction between the cathode and anode thereof. Thus, we have either acidic or alkaline fuel cells or electrolyzers depending on the particular electrolyte employed. These acidic or alkaline types of fuel cells or electrolyzers employ negative and/or positive charge ion conducting electrolytes through which charged ions (anions or cations) are transported. Electrolytic systems most often used in the past contained electrolytes that were aqueous or molten salt solutions. Examples of such systems include alkaline (KOH or NaOH) electrolyzers and fuel cells, molten carbonate electrolyzers and phosphoric acid fuel cells. Modern electrolyzers and fuel cells contain solid state electrolytes fabricated from organic compounds such as NAFION (perfluorinated sulfonate ionomer) which is manufactured by DuPont or are fabricated from inorganic compounds such as zirconium oxide.
Electrolytic gas concentrator devices, such as oxygen concentrators, are known in the prior art and typically include a cathode and anode electrode situated in an electrolyte, in a manner somewhat similar to the fuel cell and electrolyzer structures discussed above. In such oxygen concentrators, a source of DC voltage is typically coupled between these electrodes. These oxygen concentrators are known to employ either liquid or solid electrolytes. The nature of the selected electrolyte between the cathode and anode electrodes is such that gases other than oxygen cannot be transferred electrolytically through the electrolyte. Thus, the gas obtained at the anode is substantially pure oxygen whereas the gas obtained at the cathode is a nitrogen rich sample of air. Electrolytic oxygen concentrators capable of producing medical grade oxygen for patient use are very desirable; however, the processes and structures discussed above have a number of deficiencies which make them generally inappropriate for this application. For example, electrolytic oxygen concentrators which employ solid oxide electrolytes typically require very expensive, all solid state fabrication and employ rare earth elements. Additionally, these concentrators often operate at relatively high temperatures to assure adequate ion conductivity in the solid electrolyte. Moreover, alkaline oxygen concentrator systems are susceptible to carbonation by carbon dioxide in the ambient air. Additionally, such alkaline concentrator systems are also susceptible to contamination of the output oxygen by the hazardous alkali vapor emitted from the caustic electrolyte. Elaborate components are often required to provide electrolyte recirculation in these concentrators as well as to provide heat exchange and filtration.
For all these reasons, prior electrolytic oxygen concentrators have been very complex and generally inappropriate for low cost home and ambulatory patient oxygen use.